The present invention relates to lens system for projecting television images. Specifically, a multiple element lens system is employed using a single aspheric element for projecting images which lie on a curved image surface.
Projection lenses and projection cathode ray tube systems for magnifying and projecting television images to distant screens are known. In the projection television art, efforts have been made to provide simplified, and hence low cost, projection lenses for magnifying and projecting color television images.
It is recognized, specifically in U.S. Pat. Nos. 4,249,205 and 4,595,263, that curved images provide certain advantages when designing a projection lens. Specifically, the number of optical elements are simplified because of the relaxed requirements for correction of images which lie on a curved image surface.
Each of the above-referenced patents describes a projection lens system wherein curved television images are generated from cathode ray tubes having spherical or aspherical surfaces. Light from more than one curved image surfaces is combined with a dichroic mirror assembly, coupled either through an immersion medium or by air to a single aperture for projection.
The curved image surfaces can be magnified using a projection lens system which has one or more aspheric elements for preserving image resolution, providing a highly corrected image. The aspheric surfaces permit preservation of high resolution, while allowing higher lens speed. The production of these aspheric surfaces has typically been done by imposing the aspheric surface on spherical substrates (in most circumstancs, glass has been used for the substrate). Considerable tedious and expensive efforts are required to ashperize the spherical surfaces, enhancing the cost of the finished lens system. In U.S. Pat. No. 4,595,263, several aspheric surfaces are employed for providing correction to the curved image generated from the spherical CRT surface.
Thus, it is clear that one way to reduce the costs of such projection lens systems is to reduce the number of required aspheric surfaces, and to produce the elements at a lower cost.